Use of lap-top or notebook type personal computers or word-processors, etc., is being developed widely because of their portability. However, there is a common problem with compact type personal computers, that is, the electric power consumption. More specifically, these kinds of personal computers or word-processors, etc., use batteries as a portable power source, but these batteries allow the machine to operate normally for only two to five hours.
To deal with this, there are various measures taken for reducing the electric power consumption in this kind of personal computer.
One example for this measure is that if data is not input from the keyboard for a predetermined time, the operation of the CPU is stopped automatically and the display is turned off so as to reduce the electric power consumption. FIG. 1 shows a block diagram of a display device equipped with this kind of automatic display stop system together with a central processing unit (to be referred to as CPU hereinafter). Here, a display portion comprises a display controller or display image generating means 103 connected to a CPU 101 through a bus line 102, a first image display memory means 104 including a video memory (to be called a VRAM) and a liquid crystal display device (LCD) 107. A T1 measurement circuit 111 starts to measure a constant period of time T1 by means of a counter at the time an interrupt signal INTR 121 input into the CPU 101 changes from high level to low level. If the measurement circuit has finished counting the predetermined time from the count start without receiving a next interrupt signal, the circuit generates a count-up signal 113. With this, a display cut-off circuit 122, in accordance with the count-up signal 113 sent from the T1 measurement circuit 111, sets a signal 114 to high level to cut off the display. In other words, the measurement circuit 111 and the display cut-off circuit 122 constitute an electric power consumption mode switching means 108. Accordingly, the transmission of LCD data 115 and LCD drive signals 116 to a LCD is stopped. A display drive circuit 123, synchronized with the INTR 121 set to a high level after the display cut-off or in the course of counting, sets a signal 112 to high level, resets the counting and causes a signal 114 to be set to low level to operate the display. In FIG. 1, reference numerals 109 and 110 denote a power source and an external input means such as a keyboard, etc., respectively.
The conventional automatic display cut-off system, however, has a problem that if input has not been made for a predetermined period of time, the display is turned off, even when a user is looking at the display.
The present invention is to solve the problem and provide a personal computer or word-processor with which a user can take time to think, and which still prevents battery consumption.